Well that depends on if there is an active battery thermal management system, like the one on the volt which can heat or cool the battery pack... Active thermal management will definitely extend the life if you live in either extreme heat or cold climates.

Phoenix weather has always been hard on car batteries. That statement could be ammended to say "EV battery packs could operate reliably for 15 to 20 years except where it really hot like Phoenix, Arizona".
:^)